Effect of seepage conditions on the microstructural evolution of loess across north-west China

Summary Loess features metastable microstructure and is deemed susceptible to chemical contaminant permeation. However, studies on the loess permeability evolution under water and chemical environments are remarkably limited. In this study, the response of the loess to the water and sodium sulfate seepages was analyzed using the temporal relationship of cations concentration, X-ray diffraction and fluorescence (XRD and XRF), mercury intrusion porosimetry (MIP), and scanning electron microscope (SEM) tests. The permeability evolution characteristics were identified, and its underlying mechanisms were revealed from aspects of the diffuse double layer (DDL) theory and physiochemical actions. The discharge of Mg2+ and precipitation of calcium carbonate, referred also to as the dedolomitization, degraded the macro permeability when subjected to the water seepage test. The salt-induced swelling, induced by the intrusion of Na+ into the DDL, caused an increase in the micropore fraction under the sodium sulfate seepage test, thereby increasing the macro permeability.


INTRODUCTION
Loess soils are widely distributed in semi-arid and arid regions and are considered susceptible to a sudden decrease in volume when subjected to wetting, referred to also as ''collapse'' (Wang, 2003;Xue et al., 2021). Microstructural deterioration, induced by chemicals, can cause a degradation of the macro mechanical properties Wang et al., 2022b;Bai et al., 2019Bai et al., , 2021aBai et al., -c, 2022. Permeability evolution starts drawing attention in recent years as it is deemed crucial in shale gas exploration and exploitation (He et al., 2019;Nygå rd et al., 2006;Boulin et al., 2013;Rezaeyan et al., 2015;Wu et al., 2020;. Wu et al. (2021) studied the permeability evolution of the original gypsum and mudstone using the triaxial compression tests and reported the main causes leading to the differences in the permeability evolution, including the failure mode and deformation character. Kou et al. (2021) investigated the failure energy and permeability evolution of fissured rock-like materials subjected to seepage pressures. The experimental results and coupling hydro-mechanical failure mechanism enhance our understanding of aspects of monitoring and controlling rock stability in the geological engineering projects in the coupling seepagestress environments. Jiang et al. (2021) experimentally studied the evolution of pore-fracture structures and mechanism of permeability enhancement in coal under cyclic thermal shock. The findings of this study showed that cyclic thermal shock could aggravate the formation and expansion of permeability pores, connect the relatively sole fracture structures, and cause damages to the microfractures toward developing an interwoven fracture network. Li et al. (2020) analyzed the fracture permeability evolution of laminated sandstones during the rupture phase in the triaxial compression process and suggested that horizontal wells of the laminated sandstone reservoir should be set in a polyline or oblique trajectory that is likely to be intersected with the formation lamina, thereby developing an appropriate perforating direction. Lyu et al. (2021) studied the variations in pore characters of sandstone and limestone in the temperature range of 25 C-600 C based on the Katz-Thompson theory. The results showed that the permeability of limestone is nearly constant from 25 C to 400 C and is rapidly increased from 400 C to 600 C and that the permeability of sandstone can be characterized as a small fluctuation from 25 C to 400 C, followed by a gradual increase from 400 C to 570 C and subsequently a rapid increase from 570 C to 600 C.
Inorganic contaminants, including a variety of harmful metals (Otunola and Ololade, 2020;Hu et al., 2021c;Schindler et al., 2021), salts (Arocena and Rutherford, 2005;Zhang et al., 2008), acids (Wen et al., 2020;Zha et al., 2021), and alkaline substances (Seleiman and Kheir, 2018;Chen et al., 2020), and so forth are a frequent pollutant. Among the inorganic contaminants, chemical contaminant-induced accident is becoming more and more frequent and threatens seriously human health (Anae et al., 2021;Silva et al., 2021;Zhang et al., 2021). Loess soil originated in the deserts of the north  and then distributed across NW China is featured with metastable microstructure (Assallay et al., 1997;Wen and Yan, 2014;Xu et al., 2018Xu et al., , 2021cShao et al., 2018;Wei et al., 2020). The cement, formed by a fibrous calcite scaffold with a clay cover, between silt particles provides a sufficient shear strength under dry conditions despite the majority of loess soil having void ratios ranging from 0.9 to 1 (Smalley and Markovi c, 2014). Such a problematic soil with very high void ratio is, therefore, considered susceptible to chemical contaminant permeation. A significant body of research conducted over the last 10 years has greatly enhanced our understanding of the microscopic characteristics and mechanical properties of natural soils under water and chemical actions Towhata, 2011, 2013;Nguyen et al., 2013;Zhang et al., 2013Zhang et al., , 2018Zhang et al., , 2020Shao et al., 2018;Lei et al., 2020;Hu et al., 2021a). The salt effect can promote the dissolution of calcite and dolomite and the cation exchange can aggravate the leaching of other cations, thereby leading to further microstructural loosening (Xu et al., , 2021a. Given that the permeability behavior of the loess is deemed of great importance for the prevention and mitigation of chemical contamination, enhancing our understanding of the effect of seepage conditions on the microstructural evolution of loess soils is in pressing need Gao et al., 2018;Hou et al., 2020;Wang et al., 2020Wang et al., , 2021. The authors had conducted a series of experiments that pave the way to the success of interpreting the correspondence of chemical actions with the mechanical properties of loess soils (Hu et al., 2021a). Notwithstanding that, our understanding of the linkage of water and chemical seepages with the microstructural evolution of loess soils and change in the macro permeability properties is still poor. These results not only limit the development of remediation technologies but also lead to an inability of ensuring environmental sustainability and human health safety. The objectives of this study are: (1) to conduct a comprehensive investigation into the permeability evolution of the loess under the water and sodium sulfate seepages, respectively, (2) to comment on the effect of pore-water chemistry on the microscale structure characteristics of the loess using the temporal relationship of cations concentration, X-ray diffraction and fluorescence (XRD and XRF) tests, mercury intrusion porosimetry (MIP) test, and scanning electron microscope (SEM) and (3) to reveal the mechanisms affecting the permeability evolution of the loess.

Water seepage test Permeability evolution
The test processes and physical property are shown in Figures S1 and S2. Figure 1A shows the relationship of permeability k versus seepage time t for the loess specimen subjected to the water seepage test. The value of k descends very quickly in the first three days of water seepage and then remains a gentle descending tendency toward the end of the water seepage test. Figure 1B shows the change in concentration for Na + , K + , Ca 2+ , and Mg 2+ during the water seepage test. In the first three days of water seepage, the change in concentration for Na + is the most significant, followed by Ca 2+ . Both Na + and Ca 2+ show a gentle descending tendency in the remaining four days. The change in concentration for Mg 2+ is almost negligible either in the first three days or in the remaining four days. At the end of the water seepage test, the concentration of the four cations is ordered as follows: Na + , Ca 2+ , Mg 2+ , and K + .
X-ray diffraction, X-ray fluorescence, and mercury intrusion porosimetry tests Figure 1C shows the XRD test results for the loess specimen before and after the water seepage test. Four notable diffraction peak changes at 9.9545 Å , 2.9945 Å , 2.9704 Å , and 2.4907 Å , respectively, are detected and identified as illite, orthoclase, dolomite, and calcite. These results demonstrate that the four minerals play parts in the water seepage test and could help reveal the mechanism affecting the permeability evolution of the loess under the effect of water seepage. Figure 1D shows the XRF test results for the loess specimen before and after the water seepage test. The XRF test results supplement the XRD test results because the formation and/or disappearance of chemical compounds may trace back to the change in diffraction peak. The concentration of sodium oxide, magnesium oxide, and potassium oxide descends after the water seepage test, whereby the concentration of silicon dioxide and aluminum oxide ascends after the test. On the other hand, the incremental and cumulative pore volume curves, derived from the MIP tests, for the loess specimen before and after the water seepage test are shown in Figure 1E. The cumulative pore volume curve for the loess after the water seepage is differentiable from that for the loess before the water seepage. The most significant changes are present in a pore size range of 0.2-32 mm. ll OPEN ACCESS Figure 1. Water seepage test (A) Temporal relationship of permeability k versus seepage time t for the water seepage test, (B) temporal relationship of cation concentration versus seepage time t for the water seepage test, (C) XRD test results before and after the water seepage test, (D) XRF test results before and after the water seepage test, and (E) MIP test results before and after the water seepage test.

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iScience 25, 104691, August 19, 2022 3 iScience Article Scanning electron microscope tests Figure 2A presents the SEM test results of the loess before and after the water seepage, and the SEM image on the left represents a magnification of 250 times, followed by a magnification of 500 times in the middle, and a magnification of 1000 times on the right. The particle morphology of the natural loess comprises granular structures. The cementation, induced by calcium carbonate and magnesium carbonate (namely ''cement'') between silt particles, promotes the typical inter-particle connection forms (including pointedge and edge-edge connections) to be developed (see the upper part of Figure 2A). A microscale structure containing the afore-said characteristics is referred to also as the ''metastable microscale structure,'' and it is featured spaced pores, inter-particle pores, and intra-aggregate pores. The intra-aggregate pores correspond to those present in between the fine-grained particles, whereas the inter-aggregate pores refer to those separating the clay-silt aggregates. The spaced pores are present in the soil skeleton and are the largest among the three types of pores. The lower part of Figure 2A shows the particle morphology of the wetted loess. It remains nearly the same in comparison with the natural loess, but the wetting-induced collapse deformation causes the spaced pores and/or inter-particle pores to become smaller than before. The analysis of pore distribution and fractal dimension D, as well as pore directionality, is shown in Figures 2B and 2C, respectively. The pore distribution can be characterized as the micropore (<2 mm) fraction increasing and the small pore (2-8 mm), mesopore (8-32 mm), and macropore (>32 mm) fractions decreasing (Lei, 1987). The value of D owing to the effect of water seepage shows a small increase. On Sodium sulfate seepage test Permeability evolution Figure 3A shows the relationship of permeability k versus seepage time t for the loess specimen subjected to the sodium sulfate seepage test. In the first one and half days of sodium sulfate seepage, the k value  iScience Article ascends rapidly. After reaching the peak, the k value first descends a little and remains nearly constant throughout the remaining four and half days. Figure 3B shows the relationship of cations concentration versus seepage time t for the loess specimen under the sodium sulfate seepage test. In contrast to the concentration of Na + under the water seepage test, the concentration of Na + under the sodium sulfate seepage test elevates sharply in the very beginning and remains approximately constant throughout the rest of seepage time t. Furthermore, the concentration of Ca 2+ and Mg 2+ descend in the first three days of sodium sulfate seepage and shows a little change afterward. Moreover, the concentration of K + presents a negligible change throughout the whole test.
X-ray diffraction, X-ray fluorescence, and mercury intrusion porosimetry tests Five distinct diffraction peak changes, derived from the XRD test results, present at 9.9545 Å , 2.9945 Å , 2.9704 Å , 2.7037 Å , and 2.4907 Å , respectively, are detected and identified as illite, orthoclase, dolomite, gypsum, and calcite (see Figure 3C). The XRF test results show that the concentration of sodium oxide ascends after the sodium sulfate seepage test. Although the concentration of magnesium oxide, potassium oxide, silicon dioxide, and aluminum oxide descends after the seepage test (see Figure 3D). The cumulative pore volume curve for the loess subjected to the sodium sulfate seepage behaves not similar to that for the loess subjected to the water seepage (see Figure 3E). When under the sodium sulfate seepage, the pore sizes <8 mm show a more significant change.
Scanning electron microscope tests Figure 4A shows the SEM test results of the loess under the effect of sodium sulfate seepage. The particle morphology does not show a substantial change in comparison with that of the wetted loess, indicating that sodium sulfate environments have a minimal impact on the particle morphology. Furthermore, the particle aggregation becomes more significant compared to that of the wetted loess, most likely because of the improvement in cementation. The edge-edge or point-edge connections, therefore, take the lead as the main inter-particle connection form. The pore distribution indicates that the micropore (<2 mm) pore fraction increases and compared to the water seepage test, the small pore (2-8 mm), mesopore (8-32 mm), and macropore (>32 mm) fractions show a relatively smaller reduction (see Figure 4B). Furthermore, the D value increases from 1.26 to 1.34. On the other hand, the pore directionality can be iScience Article characterized as the more prominent vertical channel than the horizontal channel, indicating that the vertical channel controls the flow of pore fluid and can be considered the preferential channel (see Figure 4C).

Water seepage test
Among the cations, the temporal relationship of Na + concentration presents the most significant change at the beginning of the water seepage test (see Figure 1B). Albite, a rock-forming mineral, is considered the main source of Na + in the water seepage test. When exposed to weak acid environments (3H 2 O+2CO 2 ), it leaches sodium bicarbonate, kaolinite, and silicon dioxide (see Equation 1). Therefore, the maximum Na + concentration in the temporal relationship is attributed to the leachate containing sodium bicarbonate. The descending tendency appears in the latter part of the Na + concentration temporal relationship, most likely because of the ionization of sodium bicarbonate (see Equation 2). On the other hand, the temporal relationship of K + concentration descends all the way toward the test end. Orthoclase discoverable in the natural loess provides the main source of K + . The weak acid conditions also cause orthoclase to leach, and potassium bicarbonate in the leachate promotes the formation of the maximum in the temporal relationship of K + concentration (see Equation 3), and the descending tendency is formed with the ionization of potassium bicarbonate (see Equation 4).
The temporal relationship of Mg 2+ concentration presents a slightly ascending tendency throughout the water seepage test, whereas the temporal relationship of Ca 2+ concentration shows a descending tendency. Dolomite which is mainly consisted of Ca 2+ and Mg 2+ leaches when exposed to the water seepage, thereby discharging calcium bicarbonate and magnesium bicarbonate (see Equation 5). CaMgðCO 3 Þ 2 + Ca 2 + /Mg 2 + + 2CaCO 3 (Equation 8) The formation of the kaolinite (Al 2 Si 2 O 5 (OH) 4 ), induced by the leaching of orthoclase, can be characterized as its diffraction peak increasing from 8.4235 Å under no water seepage to 8.4307 Å under the condition of water seepage (see Figure 1C). Furthermore, the precipitation of calcium carbonate, induced by the dedolomitization, can also be recognized as its diffraction peak increasing from 2.4907 Å under no water seepage to 2.4917 Å under the effect of water seepage. Although the XRF test results provide supplements that are useful in exploring the mechanism affecting the permeability evolution of the loess under the effect of water seepage. The fraction of Na 2 O decreasing from 1.18% to 1.16% witnesses the ionization of sodium bicarbonate (see Figure 1D). Furthermore, the fraction of K 2 O decreases from 3.24% to 3.15%, which also provides testimony of the ionization of potassium bicarbonate. As discussed before, the dedolomitization discharges Mg 2+ and precipitates calcium carbonate, thereby decreasing the fraction of MgO from 2.64% to 2.57% and increasing the fraction of CaO from 16.53% to 16.68%. Given that the XRF tests are on the basis of specimens retained after the seepage tests, these results are just opposite to what has already been indicated in the temporal relationships of Mg 2+ and Ca 2+ concentration. On the other hand, the MIP test results indicate that the pore structure for the wetted loess can be characterized as the micropore fraction increasing and the small pore, mesopore, and macropore fractions decreasing (see Figure 1E). The pore distributions, derived from the SEM images, are in line with the MIP test results (see Figure 2B). Albite leaches sodium bicarbonate, and its ionization further discharges Na + and HCO 3 À . Na + can intrude the diffuse double layer (DDL) and turn the agglomerated structure into the dispersive structure. Furthermore, the water seepage can cause the swelling and dispersion of clays. The intrusion of Na + and the swelling and dispersion of clays promote the development of micropores (i.e. inter-aggregate pores) onto the cement between particles. The development of the micropores deteriorates the microstructure as well. The main cause that triggers the decrease in small pore, mesopore, and macropore fractions following the test may be ascribed to the dedolomitization. Given that the dedolomitization consumes Ca 2+ and discharges Mg 2+ as well as precipitated calcium carbonate, calcite is expected to precipitate onto the small pore, mesopore, and macropores toward decreasing the small pore, mesopore, and macropore fractions. The micropore fraction increasing and the small pore, mesopore, and macropore fractions decreasing increase the complexity of the microstructure (see Figure 2B). The pores present in the natural loess do not show a distinct orientation. Given the transformation of the inter-aggregate pores into the intra-aggregate pores when subjected to the water seepage test, the pore directionality appears to behave as similar as before (see Figure 2C). Such transformation eases the effect of inundation on the pore directionality. These results lead to a conclusion that the permeability of the loess subjected to the effect of water seepage descends all the way toward the test end (see Figure 1A).
On the whole, the Na 2 O and K 2 O fraction decreasing witnesses the ionization of sodium bicarbonate and potassium bicarbonate. When exposed to weak acid environments, sodium bicarbonate discharges Na + , and the intrusion of Na + into the double layer turns the agglomerated structure into the dispersive structure. Although the intrusion of Na + and the swelling and dispersion of clays promote the formation of micropores onto the cement, the dedolomitization, however, causes calcium carbonate to precipitate onto the small pores, mesopores, and macropores. The descending tendency of Ca 2+ concentration and the ascending tendency of Mg 2+ concentration provide testimony of the dedolomitization. It can, therefore, be summarized that the precipitation of calcite by the dedolomitization plays a leading role in modifying the micro pore fraction and degrading the macro permeability of the loess under the water seepage.

Sodium sulfate seepage test
The temporal relationship of Na + concentration shows the most notable change at the early stage of the sodium sulfate seepage test. Unlike the water seepage test, the sodium sulfate seepage causes the temporal relationship of Na + concentration to behave in an ascending tendency (see Figures 1 and 3B). This is because the involved chemical reactions, including Equations 1 and 2, reverse at high Na + concentrations. Furthermore, the diffraction peak decreasing from 8.4307 Å under the water seepage to 8.4171 Å under the sodium sulfate seepage indicates the consumption of kaolinite, induced by the reversed chemical reactions (see Figure 3C). The fraction of SiO 2 decreasing from 54.44% to 52.66% and the fraction of Al 2 O 3 decreasing from 13.22% to 12.92% provide testimony of the consumption of kaolinite (see Figure 3D).
The temporal relationship of K + concentration under the sodium sulfate seepage is rather similar to that under the water seepage (see Figures 1 and 3B). Furthermore, the sodium sulfate seepage appears to aggravate the leaching of orthoclase as the diffraction peak is reduced from 2.9898 Å under the water seepage to 2.9880 Å under the sodium sulfate seepage (see Figure 3C). The leaching of orthoclase by the sodium sulfate seepage can also be characterized as the fraction of K 2 O decreasing from 3.24% to 3.16% (see Figure 3D). On the other hand, given the dolomitization (see Equation 9), the temporal relationship of Ca 2+ and Mg 2+ shows a descending tendency (see Figure 3B). In addition to Ca 2+ and Mg 2+ , sulfate ions also take part in the dolomitization process and are believed to aggravate the leaching of dolomite. The diffraction peak decreasing from 2.9648 Å under the water seepage to 2.9458 Å under the sodium sulfate seepage gives testimony to the enhancement of the leaching of dolomite (see Figure 3C). Furthermore, the diffraction peak decreasing from 2.4917 Å under the water seepage to 2.4906 Å under the sodium sulfate seepage witnesses the reduction in the precipitation of calcium carbonate. The reduced precipitation of calcium carbonate can also be verified through the fraction of CaO decreasing from 16.53% to 15.94% (see Figure 3D). Despite that, the sulfate ions could also react with Ca 2+ in the dolomitization process toward discharging calcium sulfate (i.e. gypsum) (see Equation 10). The XRD test identifies the gypsum at 2.7037 Å . The small pore, mesopore, and macropore fractions decrease witness the precipitation of gypsum despite the dissolution of calcite by the dolomitization (see Figure 4B). Given that sodium sulfate can discharge Na + by its dissolution (Equation 11) and Na + will invade the DDL, the salt-induced swelling phenomenon is considered as the main cause leading to the micropore fraction increasing. The pores do not show a distinct orientation, although it becomes narrower compared to that of the natural loess (see Figure 4C). This is attributed to the fissures that propagate along a direction parallel to the overburden pressure under the salt-induced swelling phenomenon. The dissolution of calcite by the dolomitization not only modifies the micro pore fraction but also increases the macro permeability (see Figure 3A). iScience Article In short, the decrease of SiO 2 and Al 2 O 3 fraction provide testimony to the consumption of kaolinite. The temporal relationships of K + concentrations indicate the consumption of albite, whereas for Mg 2+ their temporal relationship indicates the leaching of dolomite by the dolomitization. Considering Ca 2+ discharged can react again with sulfate ions, their descending tendency is also attributed to the dolomitization. The small pore, mesopore, and macropore fractions decreasing witness the precipitation of calcium sulfate (gypsum), whereas the micropore fraction increasing indicates the dissolution of calcite. As a result, the dissolution of calcite by the dolomitization modifies the micro pore fraction toward increasing the macro permeability of the loess under the sodium sulfate seepage (Xu et al., , 2021a(Xu et al., , 2021b. Inappropriate disposal of waste resulting from mining activities is thought as essential for promoting the effect of pore-water chemistry (i.e. dolomitization), causing a modification of the microscale pore fraction and elevating the potential of land degradation. Land degradation refers to a loss of the productive capacity of soils and is considered to be a global challenge that influences especially rural communities and smallholder farmers through food insecurity, environmental hazards, and the loss of biodiversity and ecosystem services. As land is degraded, soil carbon and nitrous oxide are discharged into the atmosphere. This makes land degradation one of the most important contributors to climate change. Land degradation is happening at a remarkably fast pace, contributing to a dramatic decline in the productivity of croplands and rangelands around the world. The problem is particularly severe in the semi-arid and arid regions. Women and children who rely on drylands for their livelihoods are the most vulnerable to the impacts of land degradation. As a result, managing land more sustainably to reduce degradation and increasing rates of land restoration are considered of great necessity. The two ends converge to give an aggressive control of land degradation. According to the finding of this work, biomineralization technology may be used to precipitate calcium carbonate through catalyzing urea hydrolysis using the ureolytic bacteria (Hu et al., 2021b;Wang et al., 2022a, c). The precipitation of calcium carbonate not only impedes the dissolution of calcite by the dolomitization but also reduces rates of land degradation. Furthermore, biomineralization technology is also deemed effective in solidifying heavy metals where a multilayer structure of calcium carbonate binds heavy metals to precipitate onto the anionic surfaces of the cell wall. To conclude, biomineralization technology and other sustainable land management practices provide an opportunity to sustain and rebuild productive areas, increase the prospects for food security for smallholders and rural communities, and mitigate the impacts of land degradation.

Conclusions
The permeability evolution of the loess under the water and sodium sulfate seepages are studied, respectively. The microscale pore fraction modifications are in close relation with the macroscale permeability evolution. Based on the results and discussion, some main conclusions can be drawn as follows: (1) The descending tendency of Ca 2+ concentration and the ascending tendency of Mg 2+ concentration is attributed to the dedolomitization. Although the intrusion of Na + into the DDL and swelling and dispersion of clays turn the agglomerated structure into the dispersive structure and form the micropores onto the cement, the precipitation of calcite by the dedolomitization reduces the small pore, mesopore, and macropore fractions. The precipitation of calcite modifies the micro pore fraction and degrades the macro permeability of the loess under the water seepage.
(2) Ca 2+ and Mg 2+ concentrations increasing are mainly owing to the dolomitization. Ca 2+ concentration owing to the leaching of calcite would have been increased if gypsum had not been produced. Although the formation of gypsum reduces the small pore, mesopore, and macropore fractions, the hydrolysis of sodium sulfate increases the DDL thickness, referred to also as the ''salt-induced swelling'' phenomenon, thereby producing micropores. Also, the dissolution of calcite by the dolomitization modifies the micro pore fraction toward enhancing the macroscale permeability of the loess under the sodium sulfate seepage.
(3) The XRD and XRF tests provide testimony of the physicochemical reactions and are useful in exploring the mechanisms affecting the permeability evolution of the loess under the water and sodium sulfate seepages. While the MIP and SEM tests mainly aim to stand out the modifications of pore fraction which determine the macroscale permeability evolution. Although the present work is based upon indoor experiments, its findings highlight the effect of water and sodium sulfate seepage conditions on the microstructural evolution of loess, providing critical insights into land degradation prevention and control in semi-arid and arid regions around the world.

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iScience 25, 104691, August 19, 2022 9 iScience Article Limitations of the study In this study, the authors conducted a comprehensive investigation into the permeability evolution of the loess under the water and sodium sulfate seepages, respectively, commented on the effect of pore-water chemistry on the microscale structure characteristics of the loess using the temporal relationship of cations concentration, XRD and XRF tests, MIP test, and SEM, and revealed the mechanisms affecting the permeability evolution of the loess. However, the permeability evolutions and the microscale structure characteristics of the loess under acid and alkaline environments have not investigated yet. Further works in relation to the said inadequacies are ongoing and the results would be presented in another paper.

STAR+METHODS
Detailed methods are provided in the online version of this paper and include the following:

ACKNOWLEDGMENTS
This article is based upon work supported by the Shaanxi Educational Department under Grant No. 2020TD-005 through the innovative ability support scheme.